1. Field of the Invention
The present invention relates to a vessel for holding molten metal and supplying the same to a casting die after filtering impurities contained in the molten metal.
2. Description of Related Art
A vessel for holding molten metal therein and for filtering impurities contained in the molten metal is disclosed in JP-A-7-71880. Impurities such as metal oxides or other foreign particles are removed from the molten metal before the molten metal is supplied to a casting die. The vessel disclosed in the above-publication is briefly shown in FIG. 8 attached hereto.
The vessel 9 includes a melting chamber 97 and a reservoir chamber 98. A metal ingot is supplied to the vessel from an opening 99 and melted in the melting chamber. A burner 91 is submerged in the molten metal 2. An impurity separator 92 is also submerged in the molten metal and blows out inert gas therefrom to bring impurities contained in the molten metal on the surface of the molten metal. A ceramic filter 93b held by a hollow filter holder 93a is located at an inlet port of the reservoir chamber 98. Impurities contained in the molten metal 2, such as metal oxides and foreign particles, are filtered through the filter 93b, and clean molten metal is supplied to the reservoir chamber 98. An upper opening of the reservoir chamber 98 is covered by a cover plate 95 which is opened when the molten metal is taken out. When the cover 95 is opened, inert gas is blown from an inert gas supplier 96 to prevent the molten metal from being oxidized.
An outer periphery of the filter 93b is tightly fastened to a vessel body to prevent the molten metal from flowing through without being filtered. The top portion of the filter holder 93a is fixed to the vessel body. The filter 93b has to be replaced periodically to keep the filter meshes clean. In this filter structure, both the filter holder 93a and the filter 93b have to be replaced together, thereby making the maintenance cost high.
The filter structure may be modified by replacing the whole filter 93 with a filter plate, the outer periphery of which is fastened to a groove formed on the vessel body to prevent leakage flow of the molten metal and the top of which is sticking out above the molten metal surface. When the molten metal is aluminum, for example, the vessel body is made of a material obtained by sintering a mixture mainly composed of silicon oxide and alumina. Though the vessel body material of this kind is hard to react with the molten aluminum kept at a high temperature, it is difficult to keep the vessel body completely unaffected by the molten aluminum. Especially on the surface of the molten metal where the molten metal is easily oxidized, aluminum oxides stick to the wall of the vessel body. Once a spot of the aluminum oxides is formed on the wall, the aluminum oxides gradually grow on the wall around the spot. The filter periphery fastened to the groove formed on the vessel wall will be tightly fixed to the wall by the aluminum oxides, and the shape of the filter periphery will be deformed. As a result, it becomes difficult to replace the filter for maintenance, and moreover a new filter cannot be fastened to the groove.
Further, in the conventional vessel shown in FIG. 8, it is difficult to completely prevent oxidation of the molten metal surface when the cover 95 is open to take out the molten metal even inert gas is blown toward the surface. The molten metal is taken out from the vessel with a ladle. Therefore, the molten metal is oxidized by contacting air, and solid oxides are formed on the surface of the molten metal, which in turn are mixed into the molten metal. Thus, it is difficult, in the conventional vessel, to supply clean molten metal to the casting die.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a molten metal vessel in which impurities contained in the molten metal are effectively removed. Another object of the present invention is to provide an improved structure of a filter, so that the filter can be easily replaced for its maintenance. Further object of the present invention is to provide a molten metal vessel from which clean molten metal is effectively supplied to a casting die without being oxidized.
According to the present invention, a molten metal vessel includes a first chamber in which molten metal is contained, a filtration chamber having a pair of filters for removing impurities, and a second chamber in which the filtered molten metal to be supplied to a casting die is reserved. The molten metal flows from the first chamber to the second chamber through the filtration chamber. Impurities such as metal oxides and foreign particles are removed from the molten metal in the filtration chamber.
The molten metal may be supplied from a separate melting furnace to the first chamber, or solid metal may be melted in the first chamber. Preferably, a burner is submerged in the molten metal contained in the first chamber, and solid metal is dipped in the molten metal to be melted in the first chamber. By using the submerged burner, oxidation of the molten metal can be suppressed to a lower level. A pair of ceramic filters are disposed in the filtration chamber so that the impurities are filtered twice. Preferably, cylindrical filters having a larger filtering surface than plate-shaped filters are disposed in the filtration chamber. A pair of cylindrical filters are positioned laterally or coaxially with each other. The filtration chamber is preferably intercepted from the atmosphere to prevent molten metal oxidation, and an upper space above the molten metal surface is filled with inert gas. A molten metal supply pump may be disposed in the second chamber to supply the filtered clean molten metal to a casting die. The supply pump is intercepted from outside air to avoid oxidation of the molten metal. The molten metal in the second chamber is sucked into the supply pump from underneath the molten metal surface not to suck the impurities if such is afloat on the surface.
Bottom ends of the cylindrical filters abut the bottom of the filtration chamber so that no molten metal flows without passing through the filters. Preferably, the bottom ends of the cylindrical filters are fastened to grooves formed on the bottom of the filtration chamber. Since the bottom ends of the cylindrical filters are attached to the bottom of the filtration chamber where no metal oxides are developed, the cylindrical filters are easily replaced with new ones for the maintenance purpose.
Preferably, an impurity separator is disposed upstream of the filters so that impurities developed in the first chamber are removed from the molten metal before the molten metal reaches the filters. The impurity separator is dipped in the molten metal to supply inert gas bubbles which attract impurities contained in the molten metal and make them afloat on the surface of the molten metal. It is preferable to position one filter having a larger mesh size upstream of the other filter having a smaller mesh size to effectively screen the impurities and to prolong the life of the filters. Preferably, a rod-shaped heater for keeping the molten metal temperature in a desired range is disposed in the center of cylindrical filters, or at least in the center of one of the filters.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.